Traditionally, a communication link between computer systems is enabled and synchronized after the link is brought on-line. Current is run over the link at all times, leaving the lines up, running, synchronized and prepared to receive data at any time. Consequently, a receiver at one end of such a communication link never listens to a quiesced line (one over which no current flows). Broken wires in such system are detected by determining when a signal fails to reach its destination. By necessity, such a communications network consumes substantial power at all times, even when no messages are flowing.
In the environment of highly parallel computing systems where the computing function is distributed among a number of interconnected nodes, significant levels of power dissipation result from constantly-running communication lines. However, quiescing of lines between such nodes to conserve power is not a straight forward matter. Each node generally operates asynchronously and may have message traffic proceeding through it, simultaneously, in both forward and reverse directions. Each node, in such a system is a "master" node and is capable of initiating a message at substantially any time, irrespective of what other nodes are doing at such time. If links between nodes in such a system are quiesced, a procedure and system must be provided that enables any node to fully comprehend what its connected nodes are doing, and whether the communication links connecting the nodes are in proper operating order.
One part of a message protocol problem that needs to be addressed is that communicating devices must be able to recognize whether or not there is message activity. Commercial systems are available that can sense activity on a line. These devices are called transition detectors and their function is to notice a change in line voltage. They detect when a signal changes from low to high and from high to low. It is assumed that there is no active message traffic when a change to low is detected and the signal stays low for a period of time. Such transition detectors are not completely reliable. They can miss a first quiet period after a line has been quiesced. In addition, they can possibly interpret line noise, that occurs as a result of signals induced on a line during the quiesced period, as valid data. Furthermore, transition detectors are unable to detect broken wires or other physical interruptions in the communications network.
Systems for detection of broken wires generally run test patterns over communication links when they are idle. Thus, if a communication link fails after being tested but before it is used, the error condition will go undetected until message traffic is attempted.
When a full-duplex line is quiesced, a problem occurs when resynchronizing the line after power restoration. Both ends of a serial line must be in agreement before message traffic can be transmitted and properly interpreted. A transmitter must have a means for informing a receiver at the other end of the link that the transmitter is ready to send messages. One solution suggested in the prior art is to send an external signal from the transmitter to the receiver, which signal wakes up the receiver so that it can start receiving messages. This solution, requires additional wires to carry the signal and adds cost to the system.
The prior art teaches a number of communication systems that employ power-saving techniques. U.S. Pat. No. 4,484,028 to Kelley et al. describes a private automatic branch exchange wherein receivers automatically power-up and resynchronize upon receiving a burst of data. The system employs a polling technique to keep the links active and powers down after two non-responses to a poll. The system contemplates a master-slave relationship wherein the master's operation prevails.
In U.S. Pat. No. 4,259,594 to Fox et al., a transmission system is described which is powered-up when a message is to be transmitted and then powered down, subsequently. There is no consideration in this patent of nodal synchronization or the problems attendant thereto in a quiesced system.
U.S. Pat. No. 4,654,656 to Deaver et al. teaches a communication network wherein devices are capable of being manually switched to an off-line condition. A message is transmitted to the network indicating that the off-line device may, for some period of time, fail to respond to polls.
U.S. Pat. No. 4,656,318 to Noyes describes a modem with power-off ring detection. When a ring occurs, the circuit obtains power and informs the host computer to power-up the modem and to begin receiving an incoming message.
U.S. Pat. Nos. 4,754,273 and 4,812,839, both to Okada et al., describe a two-wire, time-division communication system used for digital subscriber transmission. Two wires are used to connect the subscribers and enable alternate transmission and reception of digital signals in a burst ("ping-pong") manner. Okada et al. consider transmission of one message over two wires and do not consider the situation where unrelated, asynchronous messages appear on the two lines. Okada et al. also describe a detailed technique for resynchronizing the data links and indicate that any combination of a network controller and/or subscriber can activate or deactivate a link.
U.S. Pat. Nos. 4,663,539 to Sharp et al., 4,435,761 to Kimoto, and U.S. Pat. No. 4,970,506 to Sakaida et al. all describe master/slave communication system wherein the master determines when to start and stop communications. In the '539 patent to Sharp et al., the receiver and transmitters are never shut off. In the '761 patent to Kimoto, the master quiesces and reenables a communication link, whereas in the '506 patent to Sakaida et al., either the master or the slave can power up the system. U.S. Pat. No. 4,592,051 to Frizlen describes how to un-quiesce a line which has a bias current flowing therein (it is not entirely Idle). Activation is carried out through the use of binary code signals to control the system's operation.
Accordingly, it is an object of this invention to provide an inter-nodal communication system wherein a connecting communication link only carries current when there is message activity on the link.
It is another object of this invention to provide an inter-nodal communication system wherein links are resynchronized when a message starts in either direction, and wherein additional wires are not employed for such synchronizing activity.
It is still another object of this invention to provide an inter-nodal communication system wherein broken wires, in either direction, are detected before a message completes.
It is yet another object of this invention to provide an inter-nodal communication system wherein receivers are prevented from listening to line noise caused during a period when a transmitter is quiesced--thereby preventing noise from being interpreted as valid data.
It is still another object of this invention to provide an inter-nodal communication system wherein communications do not depend upon the timing of messages, and nodes may operate in an asynchronous manner.